1. Field of the Invention
This invention relates to medical testing systems in general, and to automated vascular testing methods and apparatus in particular.
2. Brief Description of the Prior Art
Vascular testing is a valuable diagnostic tool in determining the overall vascular health of a patient. Over the years, many various tests have been devised and refined to help doctors diagnose conditions and potential problems in the circulatory, or vascular, system of a patient.
For example, a commonly performed vascular test is known in the medical field as an Ankle-Brachial Index or ABI test. Briefly, the ABI test was developed to determine whether an obstruction exists somewhere in the peripheral arteries of the patient. Specifically, the testing procedure measures the systolic pressure in the patient's arm and compares it with the systolic pressure in the patient's ankle while the patient lies in the prone position. The ankle systolic pressure is then divided by the arm, or brachial, systolic pressure to yield the ABI ratio. An ABI ratio greater than 0.97 is usually considered to indicate a normal condition while ratios below 0.97 indicate that an arterial obstruction, such as calculus deposits, might exist somewhere between the patient's heart and the patient's ankle. Such obstructions are usually found in the peripheral arteries since they become continuously smaller in diameter as they continue down the leg.
Until this invention, the ABI test was usually performed manually by a technician who would first have the patient lie in the prone position. The technician would then usually place a conventional sphygmomanometer around the arm of the patient, inflate the pressure cuff while listening via a stethoscope or Doppler ultrasound for the arteries to become occluded. He or she would then slowly deflate the pressure cuff until the arteries re-opened. At that instant, the technician would then record the pressure in the cuff, usually in millimeters of mercury (mmHg), which represents the systolic pressure. Finally, the technician would repeat this procedure on the ankle of the patient. This ankle measurement sometimes posed a problem in that it is often difficult to obtain an accurate determination of when the arteries re-open, and thus the systolic pressure. Once the technician obtained such an ankle systolic pressure, he or she would then manually calculate the ABI ratio based on the two systolic pressures. The technician would also obtain pressure cuff waveforms on each ankle. This testing procedure required a fairly skilled test administrator or technician who could accurately determine when arterial occlusion and arterial re-opening occurred. Since such a determination was usually based to a large degree on the individual judgement of the technician, the test results could vary a certain amount, and were rarely repeatable.
Another common vascular test is known as the Venous Reflux test. The Venous Reflux test was developed to evaluate the competency of the valves in both the superficial and deep venous systems in a patient's legs. The test measures valvular competency by measuring the time it takes for the leg veins to refill with blood after they have been emptied. A rapid refill time indicates a problem with the valves in the leg veins.
The Venous Reflux test is also usually manually performed one leg at a time. In this test, however, the technician usually uses a photoplethysmography (PPG) transducer to optically detect the amount of blood in the capillary system of the leg of the patient, though other detection methods are sometimes used. Briefly, the technician has the patient sit on the edge of a table with the patient's foot resting on the floor. The technician then usually clips or otherwise attaches the photoplethysmography (PPG) transducer, or other such transducer, to the calf of the leg to be tested. This transducer, and its associated detection apparatus, usually had to be carefully calibrated by the technician before the start of the test to assure the accuracy of the detected data. The technician also had to make a judgment call as to whether the patient was sufficiently "rested" before the start of the test, i.e., that the leg veins were filled to capacity and in equilibrium. This was generally done by observing the output signal from the photoplethysmography (PPG) transducer to determine if the signal had reached a steady state value. If, in the judgment of the technician, it had, he or she would proceed with the test. The remainder of the test was administered by instructing the patient to perform a predetermined number of leg flex exercises at some predetermined time interval to pump the blood out of the calf veins. After these flexure exercises, the technician would watch the output signal of the PPG transducer to determine when the steady state signal returned; when it did, the time was recorded. This time is considered to be the venous refill time. Some of the prior art apparatus made a paper strip chart recording of the transducer output signal to be used by the technician as a visual aid in determining the steady state value and the time at which it occurred. However, to a large extent, the accuracy of the test still depended on the skill of the technician in determining first whether the initial steady state had been achieved, i.e., that the leg venous system was completely filled, and second in determining when the second steady state had been achieved indicating that the venous system had completely refilled.
Finally, a Maximum Venous Outflow Test, or MVO test exists to determine whether an obstruction exists in the leg veins. This test uses pneumoplethysmography (PCR), the detection of blood flow by pressure, to measure the blood flow in the leg veins. The maximum venous outflow (MVO), which volume of blood over a given period of time (two seconds is the current standard), is divided by total venous capacitance (VC), which is the total volume of blood that can be held in the leg veins to yield the MVO ratio. An MVO ratio greater than or equal to 0.5 is generally considered to be normal. MVO ratios below this value suggest that veins in the leg may be obstructed.
Procedurally, the MVO ratio has been and can be obtained with generally known procedures as follows. The technician has the patient lie prone with the leg to be tested in a level position but being slightly bent at the knee. A pressure cuff having a pressure cuff recording or PCR transducer, or other such detection means, is wrapped around the calf and slightly pressurized to provide a reliable PCR output signal or waveform. The PCR waveform represents the blood pressure in the calf arteries, and, if the patient is fully "prepared" for the test, represents the equilibrium or baseline value. Typically, this baseline value must be "zeroed" by the technician before the test is initiated, which can be done by adjusting a baseline or zeroing level while simultaneously watching the PCR waveform generated by the pressure cuff transducer. When the technician is satisfied, he or she then places a tourniquet around the upper thigh of the same leg with just enough pressure to occlude the veins but not the arteries of the leg. The technician then observes the PCR waveform, again usually presented on a strip chart, until the wave form has "plateaued," indicating that the veins in the leg have reached capacity. This process can take up to five (5) minutes depending on the patient. Once the leg veins have filled, the technician quickly releases the tourniquet to allow the excess blood trapped in the veins to escape. After the standard 2-second interval, the pressure in the calf is noted, which represents the amount of blood remaining in the calf venous system. The difference between this amount of blood remaining and the amount of blood at equilibrium (the venous capacitance) is the maximum venous outflow or MVO.
The strip chart of the PCR data is manually read by the technician to determine the venous capacitance (VC) and the maximum venous outflow (MVO). After these numbers have been read off the chart the MVO ratio is calculated by dividing the MVO by the VC. Determining the VC is usually straightforward, since it is usually fairly easy to read the value off of the strip chart. However, to determine the MVO, the technician must find the point on the chart that corresponds to the time of tourniquet release and manually scale along the time axis two (2) seconds and calculate the MVO at that point. The physical length of the strip chart can also become quite long if it is allowed to run continuously during the venous fill time. Consequently, many technicians have adopted the practice of manually turning off the strip chart recorder during the venous fill time to save paper and make the chart easier to read. However, the technician must manually turn on the strip chart recorder before untying the tourniquet or t he usable data will never be recorded and the test will have to be repeated.
The foregoing description is intended to provide a generalized background of some of the more common prior vascular tests and the procedures used to implement them. It is not intended to fully describe all of the various tests and testing procedures available in the medical community, but it does provide a general overview of methods used prior to this invention, as well as a preferred embodiment of the present invention. At best, these various vascular test procedures comprise complex procedures, some of which require a significant degree of skill, and which are performed manually or with the aid of various pieces of equipment, such as stethoscopes, tourniquets, Doppler ultrasound detectors, a watch, and the like. Such testing procedures required skilled technicians or physicians who were not only very familiar with the sometimes complex procedures, but also with the anatomical structure and the physical principles involved. These procedures have been time-consuming and require manual handling and operating the components of equipment simultaneously with making judgments relating to zeroing, starting, performing, and stopping while interpreting data. All of this complexity limits available technicians who can handle the procedures competently. Furthermore, since such tests need only be performed on a small percentage of a general physician's patients, most physicians found that it was not cost effective to employ such a skilled technician on a full time basis. Consequently, those patients requiring vascular testing were usually referred to a hospital or a specialized clinic to have the testing performed. Such hospital referrals tended to increase both the costs of the tests and the inconvenience to the patients and usually increased the time period in which diagnoses could be made, since the test results usually had to be sent back to the referring physicians.